Electronic image detectors are increasingly being used in X-ray diagnostics. Whereas even nowadays in particular individual images are still frequently recorded using X-ray-sensitive film material, particularly in the case of series of X-ray pictures it is desirable to use electronic image detectors. These firstly allow faster image sequences to be taken, on account of the shorter exposure times required and the absence of mechanical film movement, and secondly allow the recorded data to be fed directly for electronic analysis. For example, in computer tomography (CT), series of thousands of individual images have to be combined to form an overall image, e.g. a tomogram.
Electronic image detectors which are composed of a scintillator and an adjoining photodiode are often used. The photodiode supplies, as an output signal, an analog current which is proportional to the measured X-ray intensity. An A/D converter converts this current into a digital value to be fed to an analysis computer, for example as an individual pixel of an X-ray image.
X-ray images are composed of a multiplicity of pixels which have to be generated by patterning the photodiode. Each pixel is formed by the output signal from an individually patterned photodiode, which is fed to the analysis computer via in each case a dedicated measurement channel. A dedicated electrical line with its own contacts has to be provided for each measuring channel. Contact is made via contact surfaces which serve as bonding pads. Therefore, a dedicated photodiode, a measuring channel and a bonding pad, which are arranged on the surface of the image detector, are required for each pixel.
To allow X-ray images of suitable quality to be recorded, it is necessary to use image detectors with the maximum possible area and the finest possible resolution, i.e. multi-pixelated image detectors with well over 10,000 pixels. The large number of conductor tracks, contact surfaces and connections entails a high level of outlay on patterning. At the same time, a high degree of integration has to be achieved in order not to have to use an excessively large proportion of the surface of the image detector for the measuring channel lines and bonding pads. This is because this proportion of the surface has to be regarded as an insensitive dead zone of the detector surface.
Although real image detector concepts which achieve a sufficiently high degree of integration to ensure a sufficiently fine image resolution with small detector dead zones despite the problems described are known, these concepts, e.g. what are known as FD image detectors based on amorphous silicon (a-Si) or what are known as semiconductor detectors, are complex and expensive to produce and pattern. Moreover, they have a pronounced persistence, which is responsible for inadequate time recording dynamics, which is unacceptable in particular for CT image detectors. Moreover, on account of the production process, in which scintillator and photodiode are produced separately from one another as inflexible, rigid layers and are then adhesively bonded to one another, they have poor optical properties. The adhesively bonded optical transition between the layers, which are not positioned optimally on account of the lack of flexibility, has a reduced optical transparency, which has adverse effects on the absorption efficiency of the image detector. Moreover, it causes reflections and refractions, causing crosstalk between pixelated structured image detectors.
An image detector which is less complex and less expensive to produce is disclosed in WO 99/09603. This is predicated on the use of an organic photodiode as an inexpensive alternative, which can be produced without difficulty over a large area, to inorganic-based detectors. Although organic photodiodes have time recording dynamics which are sufficient for X-ray pictures, without the aid of phosphors they are insufficiently sensitive to X-radiation to be used as X-ray detectors in X-ray image detectors. Therefore, the proposed detector has only a very limited scope for use as a CT X-ray detector.